Needle-free injectors are used as an alternative to needle-type hypodermic injectors for delivering liquid drugs and other substances through the skin and into the underlying tissue. The drug is dispensed by a piston from a drug capsule at pressures high enough to pierce the skin. Typically, the drug capsule will comprise a hollow cylindrical chamber narrowing to a discharge orifice at one end, with the piston slidingly and sealingly located at the other. The piston is driven towards the orifice to dispense the drug by a dispensing member. Typically devices are powered by a variety of means, such as a spring (the spring force being supplied by a resilient element or a pressurised gas) or pyrotechnic charge. Examples of such devices are described in U.S. Pat. Nos. 5,891,086 and 5,480,381
For spring powered needle-free injectors, the spring constantly exerts a force on the dispensing member prior to use and a restraining means is required to prevent the dispensing member from moving under the force of the spring. The needle-free injector is triggered by moving the injector into a condition in which the restraining means no longer has a restraining effect, thus permitting the dispensing member to move.
It is often desirable for the triggering of the delivery of the medication to involve a very modest force, for example by the patient pressing a button or pressing the injector against the injection site. However, this low force can cause premature triggering, such as when the device is dropped, or when the cap over the injection orifice that maintains formulation stability and sterility on storage is removed.
A problem exists during the assembly of such devices—producing a device which is easy to operate, may result in a device which is also easy to accidentally trigger during manufacture and assembly. This is both wasteful and can be a safety hazard to the personnel involved.
WO 97/37705 addressed this issue by providing a device for dispensing a material or article, which comprises a spring, which provides an energy store, a dispensing member movable to effect dispensing under the force of the spring, latch means having a first position which prevents triggering of the device, a second position in which it restrains movement of the dispensing member but enables triggering, and a third position in which it permits such movement, trigger means operable by the user for moving the latch means from said second position to said third position, the first position being a safety mechanism effective before the device has been completely assembled to prevent movement of the latch means to the third position
For the embodiments of the inventions disclosed in WO 97/37705, the safety mechanism is disengaged at the end of the assembly process, using a tool inserted into the device to move the latch from the first position to the second position. with the latch being restrained from moving to the third position by an outer-ring. The triggering means, which functions by moving the latch so that it is no longer restrained by the outer-ring, is then restricted from operating by a tear-off band, which physically stops the movement of the latch with respect to the outer ring, the band being removed prior to triggering the device.
As an alternative to the above, it may be advantageous to store the devices in the first position, with the original safety mechanism engaged. The devices are stored for up to three years before use and from both a general safety and device performance perspective there are advantages to keeping the safety mechanism engaged. Then following storage, just prior to triggering, an attachment to the device is used to disengage the safety mechanism.
In addition, the orifice must be kept sealed during storage to ensure stability and sterility of the drug product. The removal of the orifice seal is preferably combined with the disengagement of the safety mechanism, to ensure the system is easy to use, and to ensure that the orifice seal is removed prior to the disengagement of the safety mechanism, so that it is not possible that the act of removing the orifice seal triggers the delivery of the drug.